This invention relates generally to pneumatic-hydraulically operated tools and more particularly to such tools which are designed for use in the installation of pull type fasteners.
In installation tools for pull type fasteners a source of compressed air is normally applied to opposite sides of a pneumatic piston and the pneumatic piston is connected to a hydraulic piston reciprocally disposed within a bore forming a part of a closed loop hydraulic circuit so that the hydraulic piston reciprocates in unison with the pneumatic piston. The hydraulic piston in turn operates to transmit hydraulic pressure to a slave or driving piston reciprocally mounted in a bore in the head of the tool and also forming a part of the closed loop hydraulic circuit. The piston in the head of the tool has apparatus associated therewith for setting a fastener in known manner in response to reciprocal movement of the head piston. The closed loop hydraulic circuit experiences extremely high pressures which can, if not controlled, result in bursting with resultant tool damage and possible personal injury. For this reason it is common practice to incorporate some kind of a pressure relief valve system in the tool to ensure that the pressure in the closed loop hydraulic circuit does not exceed a predetermined safe upper limit. Whereas these pressure relief valves have been effective in precluding bursting of the tool and consequent tool damage and personal injury, they add considerably to the complexity and cost of the tool as well as to the maintenance requirements of the tool.